It is common for an audio recording or reproduction machine to include a device for providing a visual indication of signal level. This indication might serve to show that the signal level is adequate for the sensitivity of the particular recording machine, or it might be used to show whether the signal amplitude is so large that it cannot be recorded or reproduced, as the case may be, without introducing distortion. Such a loudness or VU (volume unit) monitor may have, as its optical readout element, a simple LED or a deflectable needle.
In some circumstances, it is desirable to be able to provide more detailed information than can be given using a simple LED or deflectable needle. For example, in more sophisticated audio recording machines it is desirable to provide an indication of both average audio signal level and peak audio signal level. One type of VU meter that is able to provide both average and peak signal levels comprises a linear (rectilinear or curvilinear) array of LEDs that form a bar graph and moving dot, representing average and peak signal levels respectively. The bar and dot are shown simultaneously, so that the user of a recording machine including such a device can see immediately, without adjusting controls, both the average level and the peak level of the received audio signal.
There are commercially available integrated circuits that are suitable for driving such an array of LEDs. For example, integrated circuit type LM 3914, available from National Semiconductor Corporation, has ten output pins that can be used to provide output signals to ten LEDs and has a mode select pin whereby the circuit may be placed either in a mode for displaying a bar or a mode for displaying a dot.
In a known VU meter, the input audio signal is applied to an average AC to DC converter and to a peak AC to DC converter, in order to provide DC signals representative of the average signal level and the peak signal level respectively. The two DC signals are applied to respective logarithmic conversion circuits so as to generate DC signals representative of the average signal level and the peak signal level in decibels. The DC signal representing the peak signal level in decibels is applied to each of a first group of four LM 3914 integrated circuits operating in the dot mode, and the DC signal representing the average audio signal level in decibels is applied to each of a second group of four LM 3914 integrated circuits operating in the bar mode. Reference voltage levels are applied to the ICs such that the forty output pins from each group of four ICs are associated with respective 1 dB increments in input signal level, the two forty dB ranges covered by the two groups of four ICs being the same. The two pins (average and peak) corresponding to each increment in signal level are connected together and are connected to an LED. The resulting forty LEDs are placed in order of signal level in a curvilinear array.
In the dot mode, the LM 3914 circuit provides an output at a single pin when the input signal lies within the range associated with that pin, whereas in the bar mode it provides an output signal at all pins associated with increments in the signal level up to and including the increment in which the input signal lies. Thus, this known VU meter results in the LEDs providing an indication in the form of an illuminated bar of the average signal level and an indication, in the form of an illuminated dot, or small group of dots, of the peak signal level. Since, in a normal audio signal, the peaks are 8 dB or more above the average signal level, the bar and the dot provide distinct, and simultaneous, visual indications of the peak signal level and the average signal level.
A problem that arises with the known VU meter is that the individual ICs in each group of four ICs are associated with different signal ranges and consequently they operate over different ranges of input and load, causing poor thermal tracking. Moreover, the ICs of the second group, since normally they each drive multiple LEDs, operate at relatively high temperatures and therefore utilize a ceramic package, which is more expensive than the molded package that can be used for ICs that operate at lower temperatures.